Zeolite catalysts are widely used to promote various processes in the petrochemical industry. These catalysts have been found to be useful, for example, in alkylation, dealkylation, transalkylation, isomerization, cracking, disproportionation and dewaxing processes. Zeolites occur in nature or may be synthesized.
Zeolites are alumino-silicates having defined, ordered crystalline structures in which there are pores or cavities of a definite size. The dimensions of the pores permit some molecules to enter for adsorption and catalysis while excluding other molecules. Thus, a zeolite catalyst can be selected that will promote specific reactions while preventing others. The adsorption properties of the zeolite may be altered by treating the zeolite to add or remove molecules from the crystalline lattice.
U.S. Pat. No. 3,251,897 to Wise discloses the alkylation of hydrocarbons in the presence of a catalyst prepared from alumino-silicates having base-exchanged metal sites and/or acid exchanged hydrogen sites to enhance catalytic activity. The preferred metals are the rare earths; for example, lanthanum, cerium and praseodymium.
U.S. Pat. No. 3,631,120 to Eberly et al. discusses the treatment of crystalline zeolites with alkaline solutions and subsequent ion-exchange with ammonium salts to enhance the activity of the zeolitic catalysts.
It is known in the art that the activity of alumina-based catalysts is enhanced by fluorination. The fluoride ion is thought to replace surface oxide or hydroxide groups, and because fluorine is very electronegative, it polarizes the framework of the catalyst, thereby increasing the acidity and reactivity of the surface. (Ghosh, A. K. and Kydd, R. A., "Fluorine-Promoted Catalysts" Catal. Rev. Sci. Eng., 1985, Vol. 27, p. 539.) Certain types of fluorine treatment have also been found to increase the acidity of siliceous zeolites such as ZSM-5. Mild fluorination of ZSM-5 was found to increase the catalysts n-butane cracking ability, although severe fluorination was found to decrease it. (Lok, B. M. Goertsma, F. P., Messina, C. A., and Izod, T. P. J., Am. Chem. Soc. Conf. Preprints, 1985, Vol. 22, p. 470.)
Fluorination may be accomplished by exposing the catalyst to the vapors of fluorine-containing compounds such as F.sub.2, HF or NH.sub.4 F at elevated temperature or saturating the catalyst with an aqueous solution containing an effective amount of a fluorine-containing compound such as HF, BF.sub.3, or HBF.sub.4. The latter fluorination is usually carried out at room temperature. (Ghosh et al., P 540-541.) X-ray diffraction studies indicate that the zeolites are dealuminated during the fluoridation process. (Id. at p.545)
One zeolite found to be useful in petrochemical catalysis is mordenite, which can be found in nature or synthesized. Mordenite comprises a crystalline structure of chains of 5-membered rings of tetrahedra and has a silicon-to-aluminum ratio of about 5 to 1. It is theorized that mordenite has a series of channels having diameters of 4 to 6.6 Angstroms which are interconnected by smaller channels with diameters of about 2.8 Angstroms. One problem in the industrial use of mordenite is that large molecules, polyalkylaromatics for example, can not enter the channels and cavities of the port structure, limiting its usefulness as a catalyst.
The fluorinated zeolite catalysts discussed above do not provide the enhanced activity and stable structure needed for commercial catalysts because of unsuitable pore size distribution. Specifically, a catalyst is needed for dealkylation that is capable of dealkylating polyalkylaromatic compounds under commercial conditions to give high yields of commercially desirable products such as benzene and ethylbenzene.